The present invention relates to a luminescent display device including a transparent substrate, particularly formed of resin substrate, and a transparent electrode, an auxiliary electrode and a luminescent layer which are laminated on the substrate and also relates to a method of manufacturing such a luminescent display device.
In general, there is known an organic EL (electro-luminescent or electroluminescent) display element, which is composed of a transparent electrode in shape of stripe laminated on a transparent substrate formed of, for example, glass, a luminescent layer made of an organic compound and laminated on the transparent electrode, and a metal electrode in shape of stripe laminated on the luminescent layer so as to be perpendicular to the transparent electrode. The transparent electrode is formed as an anode and the metal electrode is formed as a cathode, and when direct current (DC) field is applied between the transparent electrode and the metal electrode, the current passes through the organic compound and, hence, the luminescent layer is emitted. The light emitted from the luminescent layer is taken out from the transparent electrode side.
A resin substrate is used in place of the glass substrate for the purpose of making the thickness of the organic EL display device thin. The use of the resin substrate can make the thickness of the substrate itself thin and hardly cracked. And an organic EL display device utilizing the resin substrate can be bent, thus being advantageous.
However, in the case of using the resin substrate, it is necessary to form a moisture-proof layer on the resin substrate because the luminescent layer formed of an organic compound is weak to moisture (water content) and the water content passes through the resin substrate.
Further, electric current passes through the organic compound laminated on the resin substrate, and when the current passes, it becomes necessary to pay attention to a resistance of the transparent electrode. That is, the metal electrode has a low resistance and, on the other hand, the transparent electrode made of an oxide has a resistance higher than that of the metal electrode. Accordingly, the resistance of the transparent electrode becomes significant as the current passes the organic compound.
When the resistance increases, there may cause a case that the adequate field is not applied to the organic compound as being apart from a connection portion to a circuit, which results in an uneven luminescence between a plurality of organic compounds, thus being inconvenient and defective. In order to solve such defect, there is provided a technology for disposing an auxiliary electrode. That is, a metal auxiliary electrode is disposed on the side of the transparent electrode so as to be conductive thereto to reduce wiring resistance on an anode side.
FIG. 6 represents steps for preparing an organic EL element on a resin substrate as a substrate. That is, a resin substrate 1 is prepared and a moisture-proof 5 is laminated on one surface of the resin substrate 1 (step S1). A transparent electrode 2 as anode is formed, in form of film, on the moisture-proof layer 5 (step S2), and the thus laminated transparent electrode 2 is then subjected to a patterning treatment or process (step S3). Next, a metal thin film is formed as an auxiliary electrode 3 on the transparent electrode 3 (step S4). This metal thin film is thereafter subjected to the patterning treatment (step S5).
It will easily be assumed that the formation of the auxiliary electrode is needed at a time of manufacturing a luminescent display device using the resin substrate. The following problem or defect will be caused, however, at the time of forming the metal thin film on the resin substrate 1 as the auxiliary electrode 3.
That is, since the resin substrate 1 is softer in material than the glass substrate (i.e. smaller Young""s module), the resin substrate 1 was largely bowed, as shown in step S4 in FIG. 6, at the time of forming the metal thin film, which adversely influences a subsequent processes or steps including the patterning treatment of the metal thin film.
Furthermore, when the metal thin film is formed on the resin substrate 1, there may cause a case that crack 9 is formed to the moisture-proof layer 5 or the layer of the transparent electrode 2, which may then be damaged of broken. In the case of breaking the moisture-proof layer 5, harmful water content will intrude into the luminescent layer, and hence, it may become difficult to preserve the luminescent layer in good state.
An object of the present invention is therefore to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a luminescent display device in which, at a time of forming an auxiliary electrode on a resin substrate, a warpage or bowing of the resin substrate can be prevented and the moisture-proof layer and/or transparent electrode is free from being damaged, and also provide a method of manufacturing such luminescent display device.
In order to achieve the above and other objects, the inventors viewed a point that inner stress always remains in a thin film formed on the resin substrate through vacuum evaporation, spattering, epitaxial growth or like process or treatment, and the warpage of the resin substrate and/or damage of the moisture-proof layer is resulted from the remaining inner stress. The inner stress "sgr"(dyn/cm2) in the metal film is a force, per unit sectional area, which is applied from one side of the sectional area to the other side thereof. The inventors also found that the above defect or damage at the time of forming the auxiliary electrode on the resin substrate could be solved by reducing the total stress S="sgr"xc3x97d(dyn/cm) ("sgr": inner stress; d: film thickness) to a predetermined value.
Taking the above matters into consideration, inventors of the subject application achieved the above and other objects according to the present invention by providing, in one aspect, a luminescent display device comprising a resin substrate having one and another surfaces, and a transparent electrode, a metal auxiliary electrode disposed to be conductive to the transparent electrode and a luminescent layer composed of an organic compound, the transparent electrode, the auxiliary electrode and the luminescent layer being laminated in form of layers on the one surface of the resin substrate,
wherein the auxiliary electrode has a total stress of {(inner stress)xc3x97(film thickness thereof)} of not more than 1.3xc3x97105 dyn/cm.
Hereunder, the reason why the total stress of the auxiliary electrode was decided to be not more than 1.3xc3x97105 dyn/cm in the above aspect of the present invention will be explained.
The relationship between the inner stress "sgr" of the auxiliary electrode and a radius of curvature r of the warpage of the resin substrate is expressed by the following equation (1) in view of balancing between a force and moment of a minute (fine) portion of the substrate:
r=Exc2x7b2/{6(1xe2x88x92xcexd)dxc2x7"sgr"}
"sgr"=Exc2x7b2/{6(1xe2x88x92xcexd)dxc2x7r}xe2x80x83xe2x80x83(1)
(r: radius of curvature of warpage of substrate; E: Young""s modulus of substrate; b: thickness of substrate; xcexd: Poisson""s ratio of substrate; d: film thickness of auxiliary electrode; "sgr": stress of auxiliary electrode)
In the case of the resin substrate, though being different in materials which form the substrates, approximately, Young""s modulus=10000 to 20000 kgf/cm2 and Poisson""s ratio=0.4 to 0.5. The inventors of the subject application evaluated warpage amount and cracks of barrier layer (moisture-proof layer) at a time of forming chromium films of the thicknesses described in the following Table 1 by using a usual resin substrate having a thickness of 0.2 mm and length of 100 mm.
As can be seen from this Table 1, in the case of using the usually known resin substrate having a thickness of 0.2 mm, it is required for the substrate to have a warpage amount of not more than 8 mm with respect to the substrate having a length of 100 mm, and on the contrary, in the case of exceeding this value of 8 mm, the moisture-proof layer (barrier layer) will be much damaged and other defects due to a large warpage will be caused in subsequent processes. According to this matter, the total stress of the thin film of the auxiliary electrode will be calculated, and in the above case, the radius of curvature r becomes 155 mm (r=155 mm). Accordingly, the total stress will be calculated by applying this value to the above equation (1). For example, in the case of r=155 mm, E=15000 gf/cm2, xcexd=0.5, 1 kgf≈106 dyn/cm2,
"sgr"xc2x7d(dyn/cm)={15000xc3x979.8xc3x97105xc3x97(2xc3x9710xe2x88x922)2}/{6xc3x97(1xe2x88x920.5)xc3x9715.5}=1.5xc3x979.8xc3x974xc3x97105/(6xc3x970.5xc3x9715.5)=1.26xc3x97105≈1.3xc3x97105
By forming the metal film so that the total stress becomes less than the above calculated value, the auxiliary electrode in form of metal film can be formed on the resin substrate without problem. Further, the inner stress may be a tensile stress in a pulling direction of a force acting to an object through a unit sectional area perpendicular to the substrate surface (i.e. a stress applied at a time of a film being shrunk) or may be a compressive stress in a pushing direction thereof (i.e. a stress applied at a time of a film being spread).
In preferred embodiments of the above aspect, a moisture-proof layer is disposed between the resin substrate and the transparent electrode so as to shut out moisture. The auxiliary electrode is disposed between the resin substrate and a portion of the transparent electrode.
A layer of metal oxide may be formed between the resin substrate and the auxiliary electrode.
In a conventional art, as shown in steps S2 to S5 of FIG. 6, the transparent electrode 2 is formed in form of film and then patterned, and thereafter, the auxiliary electrode 3 is formed and then patterned so as to contact the side surface of the transparent electrode 2. In such steps, in a case where the transparent electrode 2 is formed at a high temperature of more than 200xc2x0 C. for a glass substrate, no problem will be provided, but the transparent electrode 2 formed at a low temperature for a resin substrate is eroded by an etching solution at the patterning treatment of the auxiliary electrode 3 such as shown in FIG. 8. Therefore, the surface of the transparent electrode 2 is made coarse, which influences the luminescence condition of the luminescent layer.
This defect can be overcome by the present invention, in which the transparent electrode is formed and patterned after the formation and patterning of the auxiliary electrode (that is, the auxiliary electrode is disposed between the resin substrate and a portion of the transparent electrode). Therefore, the transparent electrode cannot be eroded by the etching solution at the patterning of the auxiliary electrode. For this reason, the surface of the transparent electrode can be kept smooth and fine luminescence condition can be maintained. Furthermore, since the metal auxiliary electrode is covered by the transparent electrode, the oxidation of the metal auxiliary electrode can be prevented.
Furthermore, according to the formation of the metal oxide, such as indium-tin oxide, a layer between the resin substrate and the auxiliary electrode, can provide good adhesion to the metal film of the auxiliary electrode. Further, in a case where the auxiliary electrode is directly formed on the resin substrate, there is a fear of the auxiliary electrode being peeled from the resin substrate.
In another aspect, there is provided a luminescent display device comprising a resin substrate having one and another surfaces, and a transparent electrode, a metal auxiliary electrode disposed to be conductive to the transparent electrode and a luminescent layer composed of an organic compound, the transparent electrode, the auxiliary electrode and the luminescent layer being laminated in form of layers on the one surface of the resin substrate,
wherein the auxiliary electrode is formed of silver or silver alloy in form of film having a thickness of not more than 500 nm.
In a further aspect, there is provided a luminescent display device comprising a resin substrate having one and another surfaces, and a transparent electrode, a metal auxiliary electrode disposed to be conductive to the transparent electrode and a luminescent layer composed of an organic compound, the transparent electrode, the auxiliary electrode and the luminescent layer being laminated in form of layers on the one surface of the resin substrate,
wherein the auxiliary electrode is formed of aluminium in form of film having a thickness of not more than 150 nm.
Furthermore, in a further aspect, the above and other objects can be achieved by providing a method of manufacturing a luminescent display device comprising the steps of:
preparing a resin substrate having one and another surfaces; and
laminating, on the resin substrate, a transparent electrode, an auxiliary electrode made of metal to be conductive to the transparent electrode and a luminescent layer composed of an organic compound;
wherein the auxiliary electrode is formed of metal in form of film having a total stress of {(inner stress)xc3x97(film thickness thereof)} which is not more than 1.3xc3x97105 dyn/cm.
In a preferred embodiment of this aspect, the transparent electrode is laminated after the lamination of the auxiliary electrode. The method may further comprise a step of laminating a metal oxide on the one surface of the resin substrate before the formation of the auxiliary electrode.
The auxiliary electrode is formed through either one of treatments or processes of vacuum evaporation, spattering, ion-plating or epitaxial growth and is then subjected to a patterning treatment.
According to such manufacturing method, substantially the same advantageous merits as those mentioned above can be achieved.
The nature and further characteristic features of the present invention will be made more clear from the following descriptions made with reference to the accompanying drawings.